19 



3. Epicanliet er hos Clavdina (SceJ/ger, ran Beneden 

 og Jitlin), Amaroudum (Maurice, dette aibeicle. Koivaler- 

 shijs formodning) bevist at være en udl)up;tniiiii.- af tarni- 

 ti-aktus. 



4. Du at Pizon hos Bofrglhm og- Ci/Hthia morns be- 

 skrevne udbugtninger af peribranchialsækken kan derfor 

 ikke kaldes epirardicr. Selv oni man med Piton antager, 

 at peribranchialsækken hos BotrijJlHslarreriw dannes af 

 entodermen (tarmtraktus) saa bliver derfor hverken hele 

 peribranchialsækken eller en del (f. eks. den bagerste del) 

 homolog med epicardiet, saaledes som vi tinder det hos 

 AniarouciuiH. dels fordi det er ukjendt, at lige organer hos 

 nærstaaende familier anatomisk og embryonalt har saa 

 overordentlig forskjelligt forhold, dels ogsaa fordi periln'an- 

 eiiialsækken hos BotryVus i sin helhed. som kloak etc. har 

 en fnnktion, der er væsensforskjellig fra epicardiets. 



Angaaende dannelsen af hjertet og pericardiet henviser 

 jeg alene til van Beneden og Jnlin, dels fordi jeg ikke selv 

 har anstillet indgaaende studier heraf. dels ogsaa fordi 

 dette s[)orgsmaal er af mindre betydning for vort egentlige 

 maal, en sammenligning mellem knopper og larver. Jeg 

 nævner kun, at det f. eks. a f Pizon' s senere ingaaende un-_ 

 dersogelser synes at fremgaa, at disse organer i larveudvik- 

 lingen dannes af entodermen. 



For vort maal turde det fremdeles være overHodigt 

 at beskrive, hvorledes de saaledes anlagte organer nu ud- 

 vikler sig til dannelse af den færdige Ascidielarve saaledes 

 som jeg har afbildet den her Pl. X, fig. 6, Pl. XII, fig. 9. 



Det vil være almindelig bekjendt, at larven under 

 ..metamorphosen-' sætter sig fast og gjennemgaar indgri- 

 bende forandringer. Man vil erindre, at saaledes fremforalt 

 centralnervesystemet helt omforandres, idet den larvale 

 hjernebla^re afsnores og det blivende ganglion træder i virk- 

 sonihed, at halen med dens chorda dels kastes af, dels re- 

 sorberes, at sugeskaalene etc. forsvinder. 



Den unge Ascidie frembyder derfor efter metamor- 

 phosen et billede som afbildet paa fig. 9. Pl. X. Man 

 iagttager ber ligesom hos larven. Pl. X, fig. (3, ingestions- 

 og egestionsaabningerne, endostylen, gjællespalterne, samt i 

 rig. 9 ceUuhsekappen der allerede har ojjnaaet en vis mæg- 

 tighed. Dennes dannelse foretrækkor jeg at omtale i næste 

 kapitel. 



3. The epieardium in ClaveJinu (Seeliger, ran Bene- 

 den and JaJin), and Aniaroucinm {Maurice, the present 

 paper, Kowalerskg's supposition). is proved to be an eva- 

 gination of the alimentary canal. 



4. The evaginations of the peribranchial cavity des- 

 criijed by Pizon in BotryUns and Cgnthia moras cannot 

 thei"efore be callod epicardia. Even if we assunie with 

 Pizon that the peribranchial cavity in BotrgUus larva is 

 formed from tiie endoderm (alimentary canal), neither the 

 whole nor a part (e. g. the hindmost part) of the peri- 

 branciiial cavity therefore becomes homologous with the 

 epieardium as \ve tind it in Amanmcium, partly because 

 similar oi'gans in nearly-allied families have never been 

 knovvn to have eonditions so exceedingly different, anatomi- 

 cally and embryonally; i)artly because the peribranchial 

 cavity in Botrgllus, in its entirety, as cloaca, etc. has an 

 essentially different function to that of the epieardium. 



Coucerning the formation of the Jieart and the jjeri- 

 cardium, I will only refer to van Beneden and JuJin, partly 

 because I have not myself made a thorough study of the 

 subjcct, and partly because this qucstion is of minor im- 

 portance to onr main oljject, viz. a coraparison between 

 buds and larvæ. I will only mention that from Pizon^s 

 more recent. minute investigations, for iustance. it would 

 appear that in the larval development, these organs are 

 formed from the endoderm. 



It Avill also be supertiuous to our subject to describe 

 how the organs thus commenced now develope to form the 

 fully-developed Ascidian larva such as I have represented 

 it here (Pl. X, fig. 6; Pl. XII. fig. 9). 



It is generally known that during nietamorphosis, the 

 larva becomes attached, and undergoes radical changes. 

 It will be remembered that first of all the central org.ans 

 of the nervous system are completely changed, the larval 

 cerebral vesicle being constricted, and the permanent gang- 

 lion coming into activity; that the tail with its chorda is 

 partly east, partly absorbed; and that the sucking-discs, 

 etc. disappear. 



The young Ascidian presents therefore an appearance 

 such as is figured on PI. X, fig. 9. Here, as in the larva 

 (Pl. X. fig. 6). may be observed the branehial and atrial 

 orifices, the endostyle, the branehial stignmta, and, in fig. 

 9, the ceUuhse tunic, which has already attained a certain 

 importance. The formation of this organ I prefer to treat 

 of in the next chapter. 



r 



